1. Field of the Invention
This invention pertains to the cancelling of interference in a Code Division Multiple Access (CDMA) multibeam-antenna communication system.
This invention also pertains to a Direct Sequence Code Division Multiple Access (DS-CDMA) digital mobile radio system.
2. Description of the Related Art
In a CDMA system, interference between user channels is the main factor that limits channel capacity and degrades transmission quality. Multibeam-antenna CDMA systems, now in the research and development stage, are also subject to beam-signal interference by beam overlapping.
Many types of interference cancellers have been proposed so far for the CDMA system to reduce interference from other user channels caused by the cross correlation of spread codes and to enhance the signal-to-interference ratio (SIR). The most promising interference canceller has several stages of generating an interference replica signal and subtracting it from a received signal.
FIG. 1 shows a two-stage interference canceller as an example of such multistage interference canceller. In FIG. 1, Stage 1 has a replica signal generator section (41) and an interference removal section (42), and Stage 2 also has a replica signal generator section (43) and an interference removal section (44). Each replica signal generator section (41, 43) consists of interference canceller units (46) (ICU11 to ICU1K, ICU21 to ICU2K) and an adder (47). Each interference removal section (42, 44) consists of a polarity inverter (48) and an adder (49). The removal section receives an interference replica signal from the polarity inverter (48) and adds it to the received signal. A RAKE receiver (45) consists of user receivers (50) (Recl to RecK).
The interference canceller unit (46) has the configuration shown in FIG. 2. This figure shows an example of a four-finger configuration. The finger section of the preceding stage consists of despreaders (51), adders (52), channel estimation units (53), and multipliers (54). The finger section of the next stage consists of multipliers (57), adders (58), and spreaders (59) which carry out spread modulation of the signals. The interference canceller unit also contains combiners (55, 60) and a decision unit (56) coupled to combiner (55).
A received signal or a signal from the preceding stage is input to the despreader (51) in the finger section of the preceding stage corresponding to the delay profile. The spreader demodulates the input signal using a spread code, and the adder (52) adds the signal to a symbol replica signal received from the preceding stage. The channel estimation unit (53) estimates a channel parameter from the added output signal. The multiplier (54) multiplies the complex conjugate of the estimated channel parameter by the output signal from the adder (52). The combiner (55) synthesizes output signals from each multiplier (54) in the finger section of the preceding stage by RAKE synthesis.
After quantization of the synthesized signal by the decision unit (56), the quantized decision signal is input to the finger section of the next stage.
A decision signal from the decision unit (56) is input to the multiplier (57) of the finger section of the next stage. The multiplier (57) multiplies the decision signal by the estimated channel parameter from the channel estimation unit (53). The multiplied output signal is transferred to the interference canceller unit of the next stage as a symbol replica signal. The adder (58) subtracts the symbol replica signal and inputs the signal to the spreader (59). After spread modulation, the signal is input to the combiner (60), where the signal becomes an error signal.
In FIG. 1, the Stage 1 replica signal generator section (41) adds up error signals from the user interference canceller units (46) (ICU11 to ICU1K). The symbol replica signals S11 to S1K are input to the interference canceller units (46) (ICU21 to ICU2K) of the Stage 2 replica signal generator section (43).
The Stage 1 interference removal section (42) inverts the polarity of an output signal from the adder (47) with the polarity inverter (48). The adder (49) adds the signal to the received signal and generates output error signal e. The error signal is input to the Stage 2 canceller units (46) (ICU21 to ICU21).
The Stage 2 replica signal generator section (43) outputs symbol replica signals and error signals from the interference canceller units (46) (ICU21 to ICU2K). The adder (47) adds the error signals and inputs error signal e to the interference removal section (44). After polarity inversion by the polarity inverter (48), the adder (49) adds the error signal to the one from Stage 1 to produce a new error signal e.
This error signal e and the symbol replica signals S21 to S2K are input to the user receivers (50) Recl to RecK in the RAKE receiver (45) to regenerate a user symbol. This allows a signal to be received without interference between user channels.
FIG. 3 shows a multibeam-antenna system which contains several antennas (61-1 to 61-N), a beam former (62), and a receiver (63). FIG. 3 shows the main part of a multibeam-antenna system applied to a base station for CDMA communications system. The beam former (62) has the configuration outlined below. Signals received by the antennas (61-1 to 61-N) are amplified, detected, and converted from analog into digital (X1 to XN).
The N signals from X1 to XW are multiplied by the conversion coefficients from Wl,l to WN,M. An adder (64) then adds the signals to the M beam signals (B1 to BM). In other words, the beam former (62) converts the N signals received by the N antennas (61-1 to 61-N) to M beam signals.
FIG. 4 shows a system configuration in which the interference canceller is applied to a multibeam-antenna CDMA system. Interference cancellers (65) are prepared for beam signals B1 to BM outputted from the beam former (62) to cancel interference between user channels for each beam. The symbol replica and error signals from the interference cancellers are input to the receiver section, which is not illustrated here. The receiver section regenerates user data by RAKE combining and sends the data to the network.
As mentioned above, preparing a multistage interference canceller for each beam eliminates interference between users even in a multibeam-antenna CDMA system. If multiple beam antennas are used, however, beams partially overlap each other. This overlapping generates interference between signals and degrades the reception characteristic. Even a multistage interference canceller cannot eliminate interference between beam signals. To eliminate this kind of interference, we need an extra mechanism, but adding it will increase circuit scale and cost.